1. Field of the Invention
In a process for casting a Cr--Ni type stainless steel to a cast strip having a sheet thickness of not greater than 10 mm and cold rolling the cast strip to a thin sheet product, the present invention relates to a method for producing a Cr--Ni type stainless thin sheet having an excellent surface quality.
2. Description of the Related Art
Recently, a technology for directly obtaining a cast strip having a sheet thickness of not greater than 10 mm from a molten steel by casting has been developed, and experiments on an actual production scale have been carried out. According to this new technology, a hot rolling step can be simplified or omitted.
Slabs having a thickness greater than 100 mm have been hot rolled in the past in a hot rolling mill which uses enormous quantities of energy. If the hot rolling step can be simplified or omitted, it not only reduces the production cost but is also preferable from the aspect of protecting the environment. Hereinafter, a process including the step of producing a cast strip having a sheet thickness of not greater than 10 mm from a molten steel will be referred to as the "new process", and a process including a step of hot rolling a slab to a cast strip will be referred to as the "existing process".
Conventionally, when a Cr--Ni type stainless steel cold rolled strip typified by an 10% Cr-8% Ni steel is produced by the new process, skin coarsening (called "orange peel" or "roping") occurs on the surface of the product.
According to an article described in the transactions of the Japan Iron and Steel Association, entitled "Materials and Processes" (CAMP-ISIJ), Vol. 1 (1990), p. 770, the surface quality of a SUS304 thin sheet produced by the new process becomes deteriorated. This article reports than skin coarsening (roping) similar to orange peel occurs on the surface of the cold rolled sheet due to coarse crystal grains appearing in the material before finish cold rolling. The article describes further that in order to prevent this problem, the following two measures for refining the crystal grains of the material before finish cold rolling are effective:
1) To apply hot rolling and annealing to a cast strip. For example, hot rolling is conducted at a reduction ratio of 16% at 1,200.degree. C., and solution heat treatment is conducted at 1,150.degree. C. for 1 minute.
2) To apply cold rolling twice with intermediate annealing to the cast strip. For example cold rolling is conducted to a reduction ratio of 10% at a room temperature and, after intermediate annealing, finish rolling is than carried out.
On the other hand, an article of "Materials and Processes" (CAMP-ISIJ), Vol. 4 (1991), p. 996, describes that surface skin coarsening (roping) of a cold rolled sheet can be improved by applying skin pass rolling to a high reduction ratio, but because mechanical properties of the sheet, particularly elongation, drop, components must be adjusted so as to attain gamma-phase unstable components, that is, to attain a high Md 30. The article describes further that when Md 30 is 30.degree. C. and skin pass rolling is conducted to a reduction ratio of 1%, roping and mechanical properties (elongation) can be improved compared to those of the sheets produced by the existing process.
Another article in "Materials and Processes" (CAMP-ISIJ), Vol. 4 (1991), p. 997, describes that surface skin coarsening (roping) of the cold rolled sheet can be improved by a chemical component design which increases the delta-ferrite quantity and makes the gamma-phase unstable.
Japanese Unexamined Patent Publication (Kokai) No. 63-421 describes that a thin sheet having a small anisotrophy and excellent corrosion resistance can be produced by hot rolling a cast strip having a sheet thickness of not greater than 10 mm at a temperature not lower than 800.degree. C. to not higher than 50% reduction and then coiling the rolled sheet at a temperature not higher than 650.degree. C. However, this prior art technology is directed to prevent the drop of anisotrophy due to hot rolling and for this reason, it restricts the hot rolling reduction ratio to not higher than 50% and conducts hot rolling for shaping. It is therefore believed that the thin sheet produced by this prior art technology is not free from the occurrence of roping.
Japanese Unexamined Patent Publication (Kokai) No. 2-133528 describes that when hot rolling is conducted at a temperature not lower than 900.degree. C. to a reduction ratio of not higher than 60%, the structure of the cast strip is recrystallized and roping can be improved. This prior art reference describes that roping can be improved irrespective of annealing after hot rolling, but describes only that cooling is carried out at a cooling rate of at least 50.degree. C./sec within the range of temperature of 900 to 550.degree. C. as a thermal hysteresis of the cast strip after hot rolling.
The inventors of the present invention have experimentally conducted the production of the Cr--Ni type stainless steel thin sheet by employing the technology of Japanese Unexamined Patent Publication (Kokai) No. 2-133528 described above, but have failed to improve the surface quality. It has thus been clarified that a complete recrystallization structure cannot be obtained by merely conducting hot rolling under the conditions of a temperature of not higher than 900.degree. C. and a reduction ratio of not higher than 60%.